Controlled alkaline sulfite pulping

ABSTRACT

A controlled process of producing sulfite pulps from wood and other cellulosic material wherein the use of sodium sulfide is avoided and the process pH, i.e., the pH in situ or the so-called hot pH, is measured during the pulping or cooking operation and the alkalinity of the cooking medium is adjusted in response to such measurement in a predetermined manner.

United States Patent 72] Inventors Otto V. Ingruber Vankleek Hill; GlennA. Allard, Hawkesbury, Ontario, both of Canada [21] Appl. No. 871,646

[22] Filed Nov. 10, 1969 [45] Patented Dec. 28, 1971 [73] AssigneeCanadian International Paper Company Montreal, Quebec, CanadaContinuation of application Ser. No. 706,154, Feb. 16, 1968, nowabandoned. This application Nov. 10, 1969, Ser. No. 87 l ,646

[54] CONTROLLED ALKALINE SULFITE PULPING [50] Field ofSearch 162/83, 86,49, 61

[56] References Cited UNITED STATES PATENTS 1,566,118 12/1925 Rawlingl62/86X 2,920,010 1/1960 Voiret 162/83 X 3,354,030 11/1967 Williams eta1. 162/86 Primary Examiner-Howard R. Caine Attorney-R. G. McClenahanABSTRACT: A controlled process of producing sulfite pulps from wood andother cellulosic material wherein the use of sodium sulfide is avoidedand the process pH, i.e., the pl-l in situ or the so-called hot pH, ismeasured during the pulping or cooking operation and the alkalinity ofthe cooking medium is adjusted in response to such measurement in apredetermined manner.

CONTROLLED ALKALINE SULFITE PULPING This application is a continuationof application Ser. No. 706,154 filed Feb. 1968, now abandoned, which isrelated to application Ser. No. 539,908 filed Apr. 4, 1966, now U.S.Pat. No. 3,471,393.

The present invention relates to the production of sulfite pulp of veryhigh strength. More particularly, it relates to sulfite pulps ofunusually high strength prepared by cooking in a medium of controlledalkalinity.

For most of its 100 year history, sulfite pulping technology has limiteditself to the very acid range and calcium base (cold pH 1.3 to 1.5).After 1930, a narrow area close to the top of the sulfite pl-l scale wasadded and applied primarily to the chemical-mechanical high-yieldpulping of hardwoods, using sodium base. A typical NSSC (neutral sulfitesemichemical) cook of this kind used a liquor of pH 9 and the pHdecreases during the cook to 7 or less. NSSC pulps are produced on alarge scale and are chiefly used for the'manufacture of container boardsand other packaging materials.

After 1950, many attempts were made to advance sulfite technology bymaking use of the intermediate range of about pH 1.5 to pH 9. Theseattempts were partly successful and resulted in the commercialestablishment of single-stage bisulfite processes with sodium andmagnesium base (Arbiso, Magnefite) and two-stage processes with sodiumbase (Weyerhaeuser, Stora, Rauma).

Alkaline pulping processes involving sulfite have been proposedpreviously and are described in Canadian, U.S. and Norwegian patents.However, in all cases, the presence in the system of sodium sulfide wasrequired to obtain the desired pulping action. For, as is well known tothose conversant with the art, it is not possible to reach levels oflignin sufficiently low for fiber separation by cooking solely insulfite or sulfitecarbonate solutions. in fact, the NSSC cook comespractically to a standstill as the pH decreases naturally to and belowthe pH 7 level.

It has been possible to increase the strength of sulfite pulps to someextent and, by the development of proper refining techniques, toincrease the yield of usable pulp from wood to very high levels. But,although many of the sulfite pulps produced excel in brightness,cleanliness, ease of beating, bleachability and yield,'none of theserecent developments has resulted in strength properties which areconvincingly near or equal to the level set by sulfate pulps, i.e.,pulps produced by cooking in a solution of sodium hydroxide and sodiumsulfide, but, by convention, based on the addition of sodium sulfateduring recovery, so denominated.

After an uncertain start, the use of the sulfate pulping processes hasexpanded greatly, particularly since .the second World War. indeed, inrecent years, the great majority of decisions regarding newinstallations in North America has been made in favor of the sulfateprocess. These decisions are based on three principal premises:

1. sulfate fibers make stronger paper or board than sulfite fibers;

2. The sulfate process includes a proven and now standard recaustisizingsystem to close the cycle of chemicals; and,

3. advanced, though more expensive, bleaching techniques are availableto raise the brightness to high levels. Nonetheless, some persistentdifficulties encountered with the sulfate process are:

l. offensive odor due to mercaptans produced by the action of thesulfide;

2. low fiberization point;

3. resistance to beating;

4. low sheet density; and,

5. low fiber flexibility, and poor printing characteristics. Then too,the bleaching of sulfate pulps presents a further difficulty.

Now, a way has been found to produce extremely strong pulps usingsulfite and alkali and avoiding the use of sodium sulfide, theingredient thought to be critical to kraft cooks (that is, the sulfateprocess) and the sulfide-sulfite process. indeed, the pulps produced bythe process of the present invention could properly be deemed kraftpulps," having in mind that the term kraft" derives from the Swedishword for strength, albeit the expression has, over the years, come to beused principally, if not exclusively, for sulfate process pulps.

The present invention therefore provides a process for the pulping oflignocellulosic material comprising the steps of comminuting thematerial, cooking the comminuted material at high temperature, andfiberizing the cooked material, the improvement consisting of cookingthe material in a sulfidefree alkaline sulfite liquor in which thealkalinity is maintained throughout the cook in the range of about pH 8to about pH 11 as measured in situ at the cooking temperature, the pHbeing maintained within the specified limits by addition of a strongbase.

Equipment for the direct measurement of the pH value during a woodpulping or cooking process has been developed in recent years. Importantelements of this type of equipment are protected by a patent granted tothe Pulp and Paper Research Institute of Canada and an improvementthereon is covered by copending U.S. Pat. Ser. No. 539,908, filed Apr.4, 1966 issued in Canada to Canadian Pat. No. 794,495 on the 10th ofSept. 1968, Sept., 1968. By means of this equipment, it is now possibleto measure and facilitate the control of the pH level (hydrogen ionactivity) during the cooking process.

Using such hot pH equipment in a recent study of the total field ofsulfite pulping, it was possible to resolve much of the confusionexisting in previousliterature regarding the effect of individualcooking variables, and to isolate the pH factor as being of directinfluence on all pulp qualities. See TAPPl, Vol. 50, pp. 597 to 614.

It was found in this work that high levels of burst, tensile, tear, andfolding strength are naturally connected with certain high levels of pH.it was also found that large variations in physical pulp properties aredue to relatively small variations in pH level.

Another relevant finding was that the tear strength, folding resistance,and intrinsic viscosity of the pulp continue to increase with increasinghot pH levels above pH 6 and that no optimum for tear strength has beenreached at hot pH 9.5. On the other hand, tensile and bursting strengthwere found to exhibit optima at hot pH 8 and these optima are coincidentwith the xylan maximum of the pulp.

These levels and optima refer to black spruce and are expected to varyto some degree if other plant species are used, due to the naturalvariations in composition, ratios, and chemistry of the carbohydratecomponents of different woods and other lignocellulosic materials. But,a behavior similar to the above for spruce has been observed with ahardwood mixture composed chiefly of poplar, birch and maple.

Another effect brought to notice after the development of hot pHmeasurement apparatus, and which is commonly overlooked in pulpingsystems, is the shift in the ionic concentration of water dueitotemperature rise. The medium in liquidphase cooks is for the most partwater, and any intrinsic changes of its ionic composition will causelarge effects on the ionic equilibria of solutes. Due to this effect,the ionic dissociation constant of water increases on heating, causingthe H or OH concentration to rise from the level of 1X10 mole per liter(pH 7 or pOH 7) at room temperature to 2X10 6 mole per liter (pH or pOH5.70) at 200 C., or about 20 times. Consequently, a sodium hydroxidesolution of pH 12 at 25 C. has a pH of 9.39 at 200 C., and the pH ofasulfate-type liquor with no wood present is found to be about 13.5 atroom temperature and only about 10 at 166 C. 1n cooks with wood chips,the actual or hot pH values are even lower because of the consumption ofsome hydroxide for the neutralization of wood acids, either natural orformed in the cooking process.

From this brief discussion, will be clear that changes of the hydrogenand hydroxyl ion concentrations in alkaline cooks have been neglected asa basic factor in pulping kinetics and pulp quality, and that theircontrol can open new areas in pulping.

This applies particularly to the sulfite cook where previously nointelligent control to compensate for the effects of temperature riseand acid neutralization during the digestion process has been known.This, however, is the key to obtaining complete cooks and pulps of veryhigh strength in sulfite cooks.

Thus, the present discovery that valuable pulps can be produced withalkaline sulfite liquors and without resort to sodium sulfide ofmechanical refinement arises out of notpreviously-available findingsconcerning the significance of hot pH," which is an actual measure ofthe hydrogen ion activity as it exists at the temperature and pressureof the digester of cooking vessel.

The following examples will serve to disclose both the procedure used inpH-controlled sulfite cooks, and significant properties of the novelpulps obtained, in accordance with the present invention.

Example I (Sulfate control cooks at low and intermediate yield) Anexperimental digester equipped with forced circulation and indirectheating was packed with commercial chips of eastern white spruce. Thecharge was presteamed for IS minutes and the prepared liquor of 83percent activity and 30 percent sulfidity was added to give 3:1liquor/wood ratio and a chemical charge of 18 percent active alkali onO.D. wood. The temperature was raised to 166 C. in 90 minutes and heldsufficiently long to give 1) a slush pulp (Cook A) of 47.2 percent yieldat a Kappa No. of 27 and 2) a refinable pulp of 63.3 percent yield at aKappa No. of 140 (cook D).

Example ll (Sulfate control cook at very high yield) With equipment,cellulosic material, liquor and presteaming the same as in example I,the temperature was raised to 166 C. in minutes. The contents were blownimmediately to give a pulp of 78.5 percent yield at a Kappa No. of 153(cook G).

Example lll (Alkaline sulfite cooks at controlled pH 8.0 to give low,intermediate, and very high yield) Using the same digester andcellulosic material as in examplesl and ll, the charge was presteamedfor minutes. Sulfite liquor of 6 percent S0 concentration and pH 8.0 wasadded to give a 3:1 liquor/wood ratio. The temperature was raised to 166C. in 90 minutes and held at this level for different time intervals togive l cook B, a slush pulp of 46.3 percent yield at a Kappa No. of 46:(2) cook E, an intermediate yield of 67.8 percent at a Kappa No. of 127;and (3) cook H, a very high-yield pulp of 76.4 percent yield at a KappaNo. of 148. Throughout these cooks, the hot pH in the digester iscontrolled at the pH 8 level by injection ofa NaOH solution.

Example lV (Alkaline sulfite cook controlled at pH 9 to give anintermediate yield) Using the same equipment as in previous examples andblack spruce, the charge is presteamed for 15 minutes. A sulfitesolution of6 percent S0 and pH 9.0 is added to give a 45:1 liquor/woodratio and the temperature is raised to [45 C. in 90 minutes. The hot pHis controlled at 9.0 during the cook by injection ofa NaOH solution. Thetemperature is held at 145 C. to give a pulp of 64.6 percent yield atKappa No. l3l (cook F).

Example V (Alkaline sulfite cook controlled at pH 9.5 to give a lowyield slush pulp) Using the same equipment and raw material as in theexamples l to III, the charge is presteamed for 15 minutes. A sulfitesolution of 6 percent total SO, and pH 9.5 is added to give a 3.5:lliquor/wood ratio and the temperature is raised to l75 C. in minutes.The hot pH is controlled at 9.5 during the cooking by injection ofa NaOHsolution into the digester. The temperature is held at C. for 90 minutesto give a pulp yield of45.6 percent at Kappa No.23 (cook C).

Pertinent analytical and test data from pulps prepared in the foregoingexamples are collected in attached table I. A comparison of the datashows that kraft strength can be easily obtained by a sulfite process atcontrolled levels of alkalinity. This indicates that it is the hydroxylion concentration, rather than the concentration or type of sulfurchemical used in the process, which determines the strength propertiesof the pulps produced.

The data show also that there are distinct advantages of the alkalinesulfite process over the common sulfate (alkaline sulfide) process athigher levels of pulp yield and particularly at the very high levelsrepresented so far by the NSSC process. The ability of sulfite to softenthe lignin binder in the middle lamella, and thus to increasefiberability, becomes a distinct advantage at the higher yield levels.The beneficial effect on fiberability is also reflected in shorterbeating times. Another advantage apparent from the data is aconsiderably higher pulp brightness at the higher levels of yield. Thisagain is of particular interest in the manufacture of boards.

An important advantage of these sulfite pulps over pulps cooked withsulfide is the ease with which they can be bleached to high brightness.A laboratory comparison under controlled conditions shows that only 3-4bleaching stages are required to surpass 90 percent brightness and thatthe pulp viscosities are very satisfactory. The brightness reversion ofthe four-stage alkaline sulfite pulps is similar to that of sixstagesulfate pulps, compared at equal yield. When compared at equal lignin,the alkaline sulfite pulps are clearly superior. These advantages whichcould have been expected from the known behavior of sulfite ligninsconsiderably strengthens the position of sulfite pulps of high strength.

An undesirable feature of the sulfate process, and of great and mountingconcern to those associated actively or passively with this pulpingprocess, is the very offensive odor of the spent liquor. The odor is dueto mercaptans which are inevitably formed in the reaction of sulfidewith methoxyl groups of lignin and hemicelluloses. The amount of odoroussubstances formed in the sulfate or similar cooking processes has beenrelated to the amount of sulfur, as sodium sulfide, charged to theprocess. The tolerance limits for mercaptans are very low, of the orderof 0.000l to 0.00l mg./m. air, and the sensation remains unpleasant downto very low levels of concentration.

Major sources of odor in a sulfate installation are the waste gases fromcooking, evaporation and black liquor combustion, but it is clear thatin order to completely eliminate the typical sulfate odor it would benecessary to seal hermetically all containers, lines and channelscarrying spent liquor or gases.

It is a significant feature of the process of this invention that theodor of the spent liquor is very weak and pleasant rather thanunpleasant. Since the cooking agents are sulfite and hydroxyl ions thereis no chance to form mercaptans in the process and the odor arises,rather, from resinous wood extractives which, by nature, have a pleasantsmell.

Thus the present invention achieves the preparation of cellulosic pulpshaving a strength equal to or, in some cases, greater than sulfate bymeans of an alkaline sulfite cooking process which is virtuallyodorless.

In a chemical recovery cycle serving the new process, odorlessconditions will prevail anywhere before the combustion process, and thefamiliar problem there will be the control of h S and S0 emission to theatmosphere and the recovery of the sulfur values.

It should be evident also that the obnoxious smell of S0 which has beena characteristic of sulfite operation of the traditional kind (acidbisulfite process with free S0 does not arise in alkaline sulfitecooking because all S0 present is held in the state of the sulfite ionS0 While the examples given in the description of this invention includealkaline sulfite cooking processes where the hot pH is controlled at aconstant level, this cannot be considered a limitation to the type of pHcontrol applied. As specific pulp qualities are developed from variouswoods or other lignocellulosic materials by the application of thistechnique to meet various end uses, it may be found advantageous tocontrol the pH during the cook along different patterns involvinghorizontal and sloped sections, and sudden steps, for obtaining optimumresults. Such modifications of the basic technique are self-evident andare therefore part and parcel of the present disclosure.

Also the use of NaOH only as a means to create an environment ofcontrolled high alkalinity and the use of water only as the solventphase of the cooking liquor cannot be considered limitations of thisdisclosure of a cooking process employing controlled alkalinity forachieving outstanding pulp characteristics, particularly strength. Withincreasing sophistication of pulping processes, and particularly withthe advent of fully closed manufacturing systems, no limitation,costwise or other, need be imposed on the pulping medium used, e.g.,solvents other than water may be considered. The alkalinities obtainablein such systems may be at least equal to or higher than those ofsolutions of NaOH or other common bases in water. The use of a solutionof a strong base in any of the alcohols would be an example.

We claimi 1. In a process for the pulping of lignocellulosic materialcomprising the steps ofcomminuting the material, cooking the comminutedmaterial at temperatures of about C. and above, and fiberizing thecooked material, the improvement consisting of cooking the material in asulfide-free alkaline sulfite liquor in which the alkalinity ismaintained throughout the cook in the range ofabout pH 8 to about pH 1 las measured in situ at the cooking temperature, the pH being maintainedwithin the specified limits by addition of a strong base.

2. A process as in claim 1 wherein the pH is maintained within the rangeof pH 9-] l by addition of alkali.

3. A process as in claim 1 wherein the cellulosic material is wood.

4. A process as in claim 1 wherein the pH is maintained within thespecified limits by addition of sodium hydroxide.

5. An alkaline sulfite pulp prepared by the process of claim 1.

2. A process as in claim 1 wherein the pH is maintained within the rangeof pH 9-11 by addition of alkali.
 3. A process as in claim 1 wherein thecellulosic material is wood.
 4. A process as in claim 1 wherein the pHis maintained within the specified limits by addition of sodiumhydroxide.
 5. An alkaline sulfite pulp prepared by the process of claim1.